Importing the public libraries caused a name conflict with dart:sky because we assume people will import dart:sky into a namespace, so I've also changed skysprites to import dart:sky into a namespace.
Assets for this game are from Galactic Guardian: https://github.com/slembcke/GalacticGuardian.spritebuilder And are used under MIT license: https://github.com/slembcke/GalacticGuardian.spritebuilder/pull/2